The present invention relates to intermetallic compounds having improved physical properties and mechanical properties, particularly ductile properties at room temperatures. In one embodiment the invention relates to aluminide intermetallic compounds, preferably nickel aluminides, and iron aluminides containing one or more compounds selected from the group consisting of CeO2 and Cr.
Aluminides are intermetallic compounds of aluminum and one or nickel or iron. These intermetallic compounds have the potential for many high temperature application but are limited due to their poor ductile performance at room temperature where the compounds are brittle and prone to spalling if it is in the form of coatings.
U.S. Pat. No. 5,837,385 issued Nov. 17, 1998 to Schaeffer et al. assigned to General Electric Company teaches nickel aluminide having improved mechanical properties. The aluminide (a stoichometric mixture of Ni and Al) is doped with a nobel metal, elemental Cr, or a MCr alloy where M is nickel, cobalt and/or iron. Further the NiAl aluminide may contain oxygen active elements such as yttrium, hafnium, cerium, zicronium and others to promote adherence of the aluminum oxide layer. The disclosure teaches that the oxygen active element may be present in an amount up to 1 atomic %. The patent teaches the use of the element rather than the oxide as required in the present invention. Further, the patent teaches that the intermetallic is formed by a diffusion process and is not formed by a coating process.
U.S. Pat. Nos. 5,756,096 and 5,698,006 issued Jun. 9, 1998 and Dec. 16, 1997, respectively, in the name of Ishiyama, assigned to Japan Atomic Energy Research Institute teach intermetallic nickel aluminides which may be doped with up to 1 weight % of Cr. The intermetallic compounds have improved high temperature strength and satisfactory oxidation resistance. The patent does not teach the presence of CeO2.
The abstract of CN 1,306,097 published Jan. 8, 2001in the name of Li Haoqin assigned to University of Qinghua teaches the addition of one or more of TiO2, CeO2, and aluminum phosphate to a monohydrargillite powder to an aluminum based ceramic coating. The abstract does not suggest the presence of Ni or Fe in the composition
The present invention seeks to provide an intermetallic aluminide composition having good resistance to oxidation, sulphidization, and carburization and has improved ductile properties at room temperatures, chemical stabilities at elevated temperature, and resistance to high temperature thermal shock.
The present invention seeks to provide an intermetallic composition comprising
(i) from 37.5 to 49.4 atomic % of Al,
(ii) from 37.5 to 49.4 atomic % of one or more compounds selected from the group consisting of iron and nickel;
(iii) from 0 to 10, preferably from 2 to 8, most preferably 4 to 7 atomic % of Cr;
(iv) from 0.2 to 5, preferably 0.4 to 3, most preferably 0.5 to 2 atomic % of Ce,
(v) from 0 to 10, preferably from 0.8 to 6, most preferably from 1.0 to 4, atomic % of O.
The present invention further seeks to provide a coating composition having the above composition.
In a further embodiment the present invention seeks to provide a metallic substrate, preferably steel, desirably carbon steel, stainless steel, heat resistant steel, and nickel based alloys coated with the above intermetallic composition.
In a further embodiment the present invention seeks to provide a method of increasing the resistance of a metallic substrate, preferably steel, desirably stainless steel to one or more of oxidation, sulphidization and carburization by applying a coating having the above composition.